The present invention relates to a substrate having on its surface catalyst nuclei, a process of forming the catalyst nuclei on the substrate, a process of electroless-plating the substrate, a process of producing a modified zinc oxide film, and a modified zinc oxide film, which are useful for formation of transparent semiconductor electrodes on glass sheets, plastic sheets, or films used for liquid crystal displays, touch-panels, or solar cells, formation of Cu circuits on printed wiring boards, and formation of electronic part circuits such as formation of Cu wiring on Si substrates used for VLSIs.
In recent years, along with a reduction in sizes and an increase in performances of portable telephones, portable terminal instruments, and note-type personal computes, the packaging density of electronic part circuits has become higher, and correspondingly such circuits have been required to be electroless-plated with no defect. To effectively electroless-plate circuits on non-conductive substrates, metal palladium (Pd) particles are made to adsorb on the non-conductive substrates before electroless plating.
The conventional Pd catalysts, however, are disadvantageous in that the particle size is large and the density of the catalysts adsorbing on a substrate is low. An electroless plating film, which is formed by making the above Pd catalysts adsorb on a substrate and electroless-plating the substrate, has a problem that an initial precipitation layer has a low nuclei density and contains a large number of defects. Accordingly, it has been expected to develop an electroless plating film with no defect in the initial deposition layer.
On the other hand, since a zinc oxide film has a general property that the resistivity is increased after the film is left in air, it is difficult to practically utilize the zinc oxide film as a transparent conductive film.
A first object of the present invention is to provide a substrate having on its surface catalyst nuclei, which allows the formation of an electroless plating film including a dense initial precipitation layer with no defect.
A second object of the present invention is to provide a process of forming catalyst nuclei on a substrate, which is capable of forming catalyst nuclei on a non-conductive portion to be electroless-plated of a substrate.
A third object of the present invention is to provide a process of electroless-plating the non-conductive portion of the substrate on which the catalyst nuclei have been formed.
A fourth further object of the present invention is to provide process of producing a modified zinc oxide film which has good optical and electric characteristics and less variation in resistivity and thereby suitably used as a transparent conductive film.
A fifth object of the present invention is to provide the modified zinc oxide film produced by the above modified zinc oxide film production process.
The present inventors have examined to achieve the above objects, and found that fine catalyst particles adsorb on a non-conductive substrate at a high density by sensitizing the substrate by using a sensitizing solution containing stannous ions (Sn2+), activating the substrate by using a first activator containing silver ions and a second activator containing palladium ions, and finally activating the substrate by the second activator containing palladium ions, and that an electroless plating film with no effect in its initial precipitation layer is obtained by electroless-plating the non-conductive portion of the substrate on which the fine catalyst particles have been formed.
The present inventors have also found that the surface of a zinc oxide film, which is formed on a non-conductive portion of a substrate by activating the surface of the substrate in accordance with the above catalyst nuclei formation treatment and electroless-plating the non-conductive portion, can be modified by dipping the film in a modifier composed of a water solution containing at least one trivalent metal cation such as In3+, A13+, Ga3+, Tb3+, Y3+, Eu3+, Bi3+, Ru3+, Ce3+, and Fe3+, whereby the surface of the film is covered with the above trivalent metal or the oxide thereof by substitution reaction and adsorption reaction of zinc and the metal, and that when the modified film is heated, the variation rate of the resistivity of the modified film after the modified film is left in an atmosphere with a temperature of 60xc2x0 C. and a humidity of 90% for 5 to 10 days becomes as very small as 120% or less of the initial resistivity, and thereby the modified zinc oxide film is effective to be used as a transparent electrode of a liquid crystal display or a touch-panel, or a transparent semiconductor for a solar cell or the like. On the basis of the above knowledge, the present invention has been accomplished.
To achieve the first object, according to a first aspect of the present invention, there is provided a substrate having a non-conductive portion to be electroless-plated, on the surface of which metal catalyst particles composed of silver nuclei and palladium nuclei each having an average particle size of 1 nm or less adsorb at a nuclei density of 2000 nuclei/xcexcm2 or more;
wherein the metal catalyst particles are produced by sensitizing the non-conductive portion by dipping the substrate in a sensitizing solution containing bivalent tin ions, activating the non-conductive portion by dipping the substrate in a first activator containing silver ions, and activating the non-conductive portion by dipping the substrate in a second activator containing palladium ions.
The average surface roughness of the metal catalyst particles may be in a range of 0.5 nm or less.
The ratio in weight of silver particles to palladium particles may be in a range of 1:10 to 10:1.
To achieve the above second object, according to a process of forming catalyst nuclei on a substrate, comprising the steps of:
preparing a substrate having a non-conductive portion to be electroless-plated;
sensitizing the non-conductive portion by dipping the substrate in a sensitizing solution containing bivalent tin ions;
activating the non-conductive portion by dipping the substrate in a first activator containing silver ions; and
activating the non-conductive portion by dipping the substrate in a second activator containing palladium ions;
whereby catalyst particles composed of silver nuclei and palladium nuclei each having an average particle size of 1 nm or less adsorb on the non-conductive portion at a nuclei density of 2000 nuclei/xcexcm2 or more.
The sensitizing step and the first activating step using the first activator containing silver ions may be repeated by several times.
To achieve the third object, according to a third aspect of the present invention, there is provided a process of electroless-plating a substrate comprising:
preparing a substrate having a non-conductive portion to be electroless-plated;
sensitizing the non-conductive portion by dipping the substrate in a sensitizing solution containing bivalent tin ions;
activating the non-conductive portion by dipping the substrate in a first activator containing silver ions;
activating the non-conductive portion by dipping the substrate in a second activator containing palladium ions; and
electroless-plating the non-conductive portion thus activated by dipping the substrate in an electroless plating solution;
wherein catalyst particles composed of silver nuclei and palladium nuclei each having an average particle size of 1 nm or less, at both the activating steps, adsorb on the non-conductive portion at a nuclei density of 2000 nuclei/xcexcm2 or more.
The electroless plating solution may be selected from the group consisting of an electroless nickel plating solution, an electroless copper plating solution, and an electroless zinc oxide plating solution.
The substrate may be a silicon substrate on the surface of which either of a Ta film, a TaN film and a TiN film is formed, and the electroless plating solution be the electroless copper plating solution.
The substrate may be a printed wiring board having a through-hole, a peripheral wall portion of which is taken as the non-conductive portion to be electroless-plated, and the electroless plating solution be the electroless copper plating solution.
The substrate may be a polycrystalline glass substrate, and the electroless plating solution be the electroless nickel plating solution.
The substrate may be a transparent substrate such as crystal, amorphous glass plate, plastic plate or plastic film, and the electroless plating solution be the electroless zinc oxide plating solution.
To achieve the fourth object, according to a fourth aspect of the present invention, there is provided a process of producing a modified zinc oxide film, comprising the steps of;
preparing a transparent substrate having a non-conductive portion to be electroless-plated;
sensitizing the non-conductive portion by dipping the substrate in a sensitizing solution containing bivalent tin ions;
activating the non-conductive portion by dipping the substrate in a first activator containing silver ions;
activating the non-conductive portion by dipping the substrate in a second activator containing palladium ions;
electroless-plating the non-conductive portion thus activated by dipping the substrate in an electroless zinc oxide plating solution, to form a zinc oxide film;
treating the zinc oxide film with a modifier composed of a water solution containing trivalent cations; and
heating the zinc oxide film thus treated;
wherein catalyst particles composed of silver nuclei and palladium nuclei each having an average particle size of 1 nm or less, at both the activating steps, adsorb on the non-conductive portion at a nuclei density of 2000 nuclei/xcexcm2 or more.
The heating step may be performed at a heating temperature ranging from 150xc2x0 C. to 700xc2x0 C.
The heating step may be performed in a heating atmosphere selected from air, a non-oxidizing gas atmosphere, and a mixed gas atmosphere thereof.
To achieve the fifth object, according to a fifth aspect of the present invention, there is provided a modified zinc oxide film which is modified from a zinc oxide film by treating the zinc oxide film with a modifier composed of a water solution containing trivalent cations, and heating the zinc oxide film thus treated,
wherein the zinc oxide film is formed by preparing a transparent substrate having a non-conductive portion to be electroless-plated; sensitizing the non-conductive portion by dipping the substrate in a sensitizing solution containing bivalent tin ions; activating the non-conductive portion by dipping the substrate in a first activator containing silver ions; activating the non-conductive portion by dipping the substrate in a second activator containing palladium ions; and electroless-plating the non-conductive portion thus activated by dipping the substrate electroless zinc plating solution, wherein catalyst particles composed of silver nuclei and palladium nuclei each having an average particle size of 1 nm or less, at both the activating steps, adsorb on the non-conductive portion at a nuclei density of 2000 nuclei/xcexcm2 or more.
The film may have a thickness of 0.005 xcexcm or more, an average visual light transmittance of 70% or more, and a resistivity of 0.1 xcexa9cm or less.
The variation rate of the resistivity of the film after the film is left in an atmosphere with a temperature of 60xc2x0 C. and a humidity of 90% for 20 days may be 120% or less of the initial resistivity.
A process of forming catalyst nuclei on a substrate at a high density according to the present invention includes the steps of sensitizing a substrate having a non-conductive portion to be electroless-plated by using a sensitizing solution containing bivalent tin ions, activating the surface of the substrate by dipping the substrate in a first activator containing silver ions, and activating the surface of the substrate by dipping the substrate in a second an activator containing palladium ions, thereby making metal catalyst particles composed of silver nuclei and palladium nuclei adsorb on the non-conductive portion of the substrate.
The substrate used for the above process should have a non-conductive portion to be electroless-plated over the surface or at a specific area of the surface. Examples of the substrates may include non-conductive materials such as glass, plastic and ceramic materials, composites thereof, and composites of the non-conductive materials and metals.
A process of forming catalyst nuclei on the non-conductive portion of the substrate and electroless-plating it preferably includes:
(1) a cleaning step of cleaning the substrate having the non-conductive portion to be electroless-plated under a conventional degreasing condition;
(2) a surface preparation step of imparting electric charges on the surface of the non-conductive portion by using a conventional surface preparation agent;
(3) a sensitizing step of sensitizing the surface of the non-conductive portion by dipping the substrate in a conventional sensitizing solution containing divalent tin ions (Sn2+);
(4) a first catalyst nuclei formation step of activating the surface of the non-conductive portion by using a first activator mainly containing silver ions;
(5) a second catalyst nuclei formation step of activating the surface of the non-conductive portion by using a second activator containing palladium ions; and
(6) an electroless plating step of forming an electroless plating film on the non-conductive portion of the substrate.
It should be noted that, in the above method, a substrate rinsing step is inserted between the continuous two steps, and the steps (3) and (4) may be repeated by several times as needed and similarly the steps (4) and (5) may be repeated by several times as needed.
Hereinafter, each of the above steps (1) to (6) will be described in detail.
The surface preparation agent may be configured as a water solution mainly containing a cationic surface active agent or cationic polymer compound in an amount of 1 to 50 g/L. The substrate may be dipped in this surface preparation agent kept at a temperature of 10 to 60xc2x0 C. for a time of 1 to 10 min.
The sensitizing solution used for the sensitizing treatment may be configured as a water solution containing bivalent tin ions in an amount of 1 to 50 g/L and having a pH of 1 to 3, which is prepared by dissolving a bivalent tin salt such as SnCl2 or SnSO4 in an acidic solution such as hydrochloric acid or sulfuric acid. The substrate to be treated may be dipped in this solution kept at a temperature of 10 to 60xc2x0 C. for a time of 10 sec to 5 min, preferably, 30 sec to 2 min.
The first activator containing silver ions used at the above step (4) may be configured as a water solution containing silver ions in an amount of 0.0001 to 0.5 mol/L, preferably, 0.001 to 0.1 mol/L.
As a supply source of silver ions, a silver salt such as silver sulfate, silver sulfite, silver nitrate, silver thiosulfate, or silver methanesulfonate may be used but not limited thereto. The activation ability of the activator mainly containing silver ions can be improved by adding bivalent metal ions thereto.
To be more specific, nickel ions, cobalt ions, iron ions, zinc ions, or copper ions may be preferably added to the activator mainly containing silver ions. The concentration range of the additional metal ions may be the same as that of the silver ions, i.e., in an amount of 0.0001 to 0.5 mol/L, preferably, 0.001 to 0.1 mol/L. In addition, sulfate ions, nitrate ions, halogen ions, or methanesulfonate ions may be used, but not limited thereto, as anion ions to the above-described silver ions. The pH of the activator mainly containing silver ions may be in a range of about 5 to 11.
The temperature of the activator mainly containing silver ions according to the present invention can be set in a wide range but may be generally set in a range of 15 to 60xc2x0 C. The dipping time in the activation treatment using the silver based activator can be suitably selected but may be generally set in a range of 10 sec to 5 min, preferably, 30 sec to 2 min.
The second activator containing palladium (Pd) ions used at the step (5) may be configured as a solution containing bivalent Pd ions in an amount of 0.01 to 1 g/L and having a pH of 1 to 3, which is prepared by dissolving a bivalent Pd salt such as PdCl2 or PdSO4 in an acid solution such as hydrochloric acid or sulfuric acid. The substrate to be treated may be dipped in this solution kept at a temperature of 10 to 60xc2x0 C. for a time of 1 sec to 5 min, preferably, 1 sec to 1 min. If the substrate is dipped in this solution for an excessively longer time, there may occur aggregation of Pd particles, which may obstruct formation of the initial dense precipitation layer.
The activation ability of the second activator can be improved by adding Pb(NO3)2, Ag2SO4 or borofluoric acid to the solution mainly containing Pd ions in a slight amount, preferably, in a range of 0.1 to 100 mg/L.
According to the present invention, the above steps (3) and (4) may be repeated by several times, preferably, two to six times, more preferably, three to four times with the rinsing step put therebetween. With this repetition of the steps (3) and (4), it is possible to certainly form a high dense catalyst layer.
The above steps (4) and (5) may be repeated by two to six times with the rinsing step put therebetween, and particularly from the viewpoint of avoiding aggregation of Pd particles, may be repeated by two to three times with the rinsing step put therebetween.
The electroless plating solution used at the above step (6) may be either of known autocatalytic type electroless plating solutions, for example, an electroless copper plating solution using formaldehyde as a reducing agent; an electroless nickel-phosphorus plating solution using sodium hypophosphite as a reducing agent; an electroless nickel-boron plating solution using dimethylamine-borane as a reducing agent; an electroless palladium plating solution; an electroless palladium-phosphorus plating solution using sodium hypophosphite as a reducing agent; an electroless gold plating solution; electroless silver plating solution; and an electroless nickel-cobalt-phosphorus plating solution using sodium hypophosphite as a reducing agent.
The electroless plating using the above electroless plating solution may be performed under a conventional plating condition corresponding to the kind of the plating solution. The plating thickness is suitably set in accordance with the application of the substrate having been subjected to the electroless plating.
As the electroless plating solution, there also can be used an electroless zinc oxide plating solution capable of depositing zinc oxide (ZnO). Such a plating solution may be configured as a solution containing a zinc salt such as zinc sulfate in an amount of 0.01 to 0.5 mol/L, preferably, 0.05 to 0.2 mol/L, and a borane based reducing agent such as dimethylamine-borane or another reducing agent in an amount of 0.001 to 0.5 mol/L, preferably, 0.01 to 0.2 mol/L, more preferably, 0.03 to 0.1 mol/L, and having a pH of about 4 to 9, preferably, about 6.5. The substrate to be treated may be dipped in this plating solution kept at a temperature of 10 to 80xc2x0 C. for a time of 5 to 120 min.
As the most preferable electroless zinc oxide plating solution, there can be used a solution containing 0.1 mol/L of Zn(NO3)2 and 0.03 mol/L of dimethylamine-borane and having a pH of 6.5. A zinc oxide film formed by using such a plating solution is advantageous in that the particle size is small, each crystal is oriented along the C-axis (0001), and the number of voids is reduced, with a result that the transparency and electric conductivity of the film are improved.
According to the present invention, metal catalyst particles of silver and palladium can be made to adsorb, by the above catalyst nuclei formation treatment, on the non-conductive portion to be electroless-plated of the substrate at a nuclei density of 2000 nuclei/xcexcm2 or more, preferably, 2000 to 5000 nuclei/xcexcm2, particularly, 2500 to 3500 nuclei/xcexcm2. In this case, the metal catalyst particle layer formed on the surface of the non-conductive portion at a high density can have an average surface roughness of 0.5 nm or less, preferably, 0.05 to 0.5 nm, particularly, 0.1 to 0.3 nm; and an average catalyst particle size of 2 nm or less, preferably, 0.1 to 2 nm, more preferably, 0.3 to 1 nm. It should be noted that the above nuclei density, average roughness, and average particle size can be measured by AFM (Atomic Force Microscope).
In the metal catalyst particles formed according to the present invention, the weight ratio between silver and palladium may be 1:10 to 10:1 preferably, 1:4 to 3:1, more preferably, 1:3 to 1:1. It should be noted that the ratio between the contents of silver and palladium can be analyzed by ESCA (Electron Spectroscopy for Chemical Analysis).
According to the present invention, fine catalyst particles can be made to adhere on a non-conductive substrate at a high density by treating the substrate using the first activator mainly containing silver and the activator containing palladium, and finally dipping the substrate in the second activator containing Pd ions.
The mechanism for making fine metal particles adhere on the non-conductive portion at a high density is not clear but may be considered as follows: namely, silver ions adsorb on the surface of the substrate at a high density preferably by repeating several times the sensitizing treatment (performed by dipping the substrate in the sensitizing solution containing tin ions) and the activation treatment (performed by dipping the substrate in the first activator containing silver ions), and palladium particles are precipitated on the surface of the substrate, on which the silver ions having adsorbed, by dipping the substrate in the second activator containing palladium ions, whereby fine metal particles made from silver and palladium adhere on the surface of the substrate at a high density by interaction between the silver ions and palladium ions.
According to the electroless plating process using the above-described catalyst nuclei formation treatment, an electroless plating film having the initial dense deposition layer can be formed without occurrence of defect. Such an electroless plating film can be effectively used in the filed of electronic parts, for example, for formation of a printed wiring board and a Cu circuit on a VLSI chip, formation of an Nixe2x80x94P underlayer for a computer hard desk, and formation of a transparent electrode for a liquid crystal display and a transparent semiconductor electrode for a solar cell.
According to the present invention, fine metal catalyst particles made from silver and palladium can be formed on a non-conductive substrate at a high density.
Incidentally, the above-described zinc oxide film formed by the electroless plating process of the present invention may be subjected to heat-treatment. The heat-treatment may be performed at a temperature of 150 to 700xc2x0 C., preferably, 200 to 650xc2x0 C., more preferably, 400 to 600xc2x0 C. for a time of 5 min to 2 hr, particularly, 10 min to 1 hr. The heating atmosphere may be either of atmospheric air, a non-oxidizing gas atmosphere such as nitrogen, helium or argon, and a mixed gas atmosphere thereof.
With this heat-treatment, the zinc oxide film can exhibit a good transparency, concretely, an average visual light transmittance of 70% or more, particularly, 80% or more, and a good electric conductivity, concretely, a resistivity of 0.1 xcexa9cm or less, particularly, 0.05 xcexa9cm or less.
Next, a modified zinc oxide film will be described in detail.
The zinc oxide film formed by the electroless plating process using the catalyst nuclei formation treatment can be modified into a zinc oxide film excellent in optical and electric properties and further in heat resistance and moisture resistance by treating the zinc oxide film with the following modifier and heating the treated film.
The modifier according to the present invention is configured as a water solution containing trivalent metal cations. Specific examples of the trivalent metal cations may include In3+, Al3+, Ga3+, Tb3+, Y3+, Eu3+, Bi3+, Ru3+, Ce2+, Fe3+. They are used singly or in combination. As anions to the trivalent metal ions, there may be used, but not exclusively, anions capable of making the trivalent metal ions water-soluble. Specific examples of the anions may include sulfate ions, halogen ions, phosphate ions, nitrate ions, acetate ions, citrate ions, lactate ions, and carboxylate ions. The trivalent metal cations may be contained in a water solution in an amount of 0.1 to 50 g/L, preferably, 0.3 to 10 g/L, more preferably. 0.5 to 5 g/L.
The pH of the trivalent metal cation containing water solution (modifier) may be in a range of 2 to 10, preferably, 3 to 8.
The modifier of the present invention may contain ammonium sulfate in an amount of 0.1 to 5 g/L, preferably, 0.5 to 2 g/L, polyethylene glycol in an amount of 0.01 to 1 g/L, preferably, 0.05 to 0.5 g/L, and L-ascorbic acid in an amount of 0.01 to 1 g/L, preferably, 0.05 to 0.5 g/L.
The treatment condition for treating the zinc oxide film with the modifier of the present invention may be suitably selected and may be generally set such that the treatment temperature is in a range of 10 to 60xc2x0 C., preferably, 20 to 40xc2x0 C. and the treatment time is in a range of 1 sec to 10 min, preferably, 5 sec to 5 min. The treatment may be performed by dipping the zinc oxide film in the modifier, or spraying the modifier on the zinc oxide film.
According to the present invention, the modifying treatment is performed by dipping the zinc oxide film in the modifier configured as a water solution containing trivalent metal cations or spraying the water solution on the zinc oxide film. With this treatment, the trivalent metal substitutes for and adsorb on zinc, so that the zinc oxide film is covered with the trivalent metal or the oxide thereof. As a result, a modified zinc oxide film having a good electric conductivity and a resistivity with less variation can be obtained. The reason why the electric conductivity of the zinc oxide film is improved is not clear but may be considered as follows: namely, the concentration of carriers is increased by using a trivalent metal as a donor element to bivalent Zn. The reason why the variation in resistivity of the zinc oxide film becomes smaller is not clear but may be considered as follows: namely, the property inherent to zinc oxide that the resistivity is increased after the zinc oxide is left in air is eliminated by covering the outermost layer of the zinc oxide film with a stable layer made from a material different in property from zinc oxide.
As a result of element analysis of the surface of the modified zinc oxide film obtained by the above-described method by ESCA (Electron Spectroscopy for Chemical Analysis), it was confirmed that the outermost layer of the zinc oxide film is covered with a trivalent metal such as In, Al, Ga, Tb, Y, Eu, Bi, Ru or Ce, or the oxide thereof.
According to the present invention, the zinc oxide film thus modified may be preferably subjected to heat-treatment.
The heat-treatment may be performed at a temperature of 150 to 700xc2x0 C., preferably, 200 to 650xc2x0 C., more preferably, 400 to 600xc2x0 C. for a time of 5 min to 2 hr, particularly, 10 min to 1 hr.
The heating atmosphere may be either of atmospheric air, a non-oxidizing gas atmosphere such as nitrogen, helium or argon, and a mixed gas atmosphere thereof.
With this heat-treatment, the variation rate of the resistivity after the film is left in an atmosphere with a temperature of 60xc2x0 C. and a humidity of 90% for 5 to 20 days can be reduced to 120% or less of the initial resistivity. Further, the zinc oxide film thus heat-treated can exhibit a good transparency, concretely, an average visual light transmittance of 75% or more, particularly, 80% or more, and a good electric conductivity, concretely, a resistivity of 0.1 xcexa9cm or less, particularly, 0.05 xcexa9cm or less. Additionally, the lower limit of the above resistivity is 1xc3x9710xe2x88x922 xcexa9cm or more. Accordingly, the zinc oxide film of the present invention having a good electric conductivity and a good transparency can be effectively used for a transparent electrode of a liquid crystal display or a transparent semiconductor electrode of a solar cell. In addition, the thickness of the zinc oxide film can be set, not limited thereto, in a range of 0.005 xcexcm or more, preferably, 0.01 to 2 xcexcm, particularly, 0.1 to 1 xcexcm.
According to the present invention, by applying the modifier and the modifying method of the present invention to an electroless zinc oxide film formed by electroless plating, a modified zinc oxide film having the above film characteristics can be relatively easily formed on a substrate having a large size and a three-dimensional free curve surface. Further, the modified zinc oxide film can be applied to a large-sized substrate such as a car window glass sheet or an architectural window glass sheet by adjusting the characteristics as needed.
The modified zinc oxide film treated with the modifier of the present invention has a reduced variation in resistivity with elapsed time.
The treatment using the above modifier also can be effectively used for a zinc oxide film formed by electroplating a substrate or the above-described electroless zinc oxide film.
In this case, as an electrolytic solution, any solution can be used so long as it can deposit zinc oxide; although a solution containing a zinc salt such as zinc nitrate in an amount of 0.01 to 0.5 mol/L, preferably, 0.05 to 0.2 mol/L, and having a pH of about 4 to 9, preferably, 6 may be preferably used. The electrolytic solution is electrified by a quantity of 0.1 to 20 coulombs, preferably, 1 to 10 coulombs per 1 cm2 of a conductive substrate by using an anode made from zinc, carbon or platinum. The temperature of the electrolytic solution is kept in a range of 10 to 80xc2x0 C.